In XR (Extended Reality) applications, physics simulations are used to power interactions like object collisions, gravity-based behaviors, and realistic movement. However, when XR physics behave inconsistently across different devices, it can break immersion, cause gameplay errors, and make debugging incredibly challenging.
This issue often stems from differences in device performance, frame rates, physics engine settings, or even hardware-specific quirks.
What Does “Inconsistent Physics” Look Like in XR?
- Objects fall faster on one headset than another.
- Interactable objects bounce or float unpredictably.
- Physics-based puzzles or movement systems break on certain devices.
- Multiplayer interactions don’t sync correctly across platforms.
- Motion or collision events fail to trigger consistently.
Why It Happens
1. Frame Rate Dependency
Some physics simulations are tightly coupled with frame rate (Update() instead of FixedUpdate()), causing different outcomes on 60Hz vs 90Hz devices.
2. Variable Time Steps
Inconsistent or high Time.deltaTime and FixedDeltaTime values can lead to non-deterministic physics behavior, especially when devices throttle performance.
3. Different Physics Engine Settings
Unity or Unreal might use different physics solvers, tolerances, or iteration counts on different platforms or build targets.
4. Floating Point Precision Errors
Portable headsets (like Quest) may use reduced precision for performance, which can subtly affect physics accuracy over large scenes.
5. Hardware Variability
CPUs and GPUs on lower-end devices might skip physics steps or simulate differently to maintain performance, leading to desynchronization.
6. Desync in Networked XR Apps
Multiplayer XR apps using physics without authoritative sync or proper interpolation will show different physics states on different devices.
Examples of Inconsistent Physics in XR
| Symptom | Root Cause |
|---|---|
| Object falls slowly in Quest but faster on PC VR | Frame-dependent gravity or force application |
| Ball bounces more or less between devices | Differing Fixed Timestep values |
| Multiplayer users see objects in different positions | Lack of networked physics syncing |
| Hand-held objects jitter when moved quickly | Poor interpolation or late physics updates |
| Sliding or rotating platforms behave unpredictably | Inconsistent solver iterations across devices |
How to Fix or Prevent XR Physics Inconsistencies
✅ 1. Always Use FixedUpdate() for Physics
Ensure forces, velocities, and rigidbody interactions occur in FixedUpdate():
void FixedUpdate()
{
rb.AddForce(Vector3.down * gravity);
}
✅ 2. Set Consistent Time Step Values
Go to Project Settings > Time and set:
- Fixed Timestep: Use a consistent value (e.g.,
0.01or0.0167) - Maximum Allowed Timestep: Prevent large time gaps on slow devices (e.g.,
0.033)
These ensure uniform physics calculations across frame rates.
✅ 3. Match Physics Settings Across Build Targets
Ensure the same Physics settings are used across platforms (Unity: Edit > Project Settings > Physics):
- Solver Iteration Count
- Collision Detection Mode
- Default Contact Offset
- Use Auto Simulation (only if needed)
✅ 4. Use Interpolation on Rigidbodies
Enable interpolation to smooth physics-driven object motion between physics steps:
rigidbody.interpolation = RigidbodyInterpolation.Interpolate;
✅ 5. Avoid Large Mass or Force Disparities
Stick to realistic ranges for mass, drag, and force values. Extreme values can lead to inconsistent simulation depending on platform precision.
✅ 6. Profile Across Devices
Use device profilers (Unity Profiler, Unreal Insights) to track physics step duration and performance. Optimize logic accordingly.
✅ 7. Deterministic Physics (Advanced)
For precision-sensitive apps (e.g., physics puzzles), consider using a deterministic physics engine or record/replay physics states instead of relying on real-time simulation.
✅ 8. Sync Physics in Networked XR
Use authoritative server logic or predictive sync systems (like Photon Fusion, Netcode for GameObjects) to ensure physics is identical across players.
Real-World Example: VR Object Grabbing Feels “Floaty” on Mobile
Cause: Force-based grabbing logic was applied in Update() instead of FixedUpdate(), leading to input lag and slow response on lower FPS devices like standalone VR headsets.
Fix: Move interaction logic to FixedUpdate() and enable rigidbody interpolation.
Debugging Checklist
- Is the physics logic in
FixedUpdate()? - Are time step settings consistent?
- Are physics materials consistent across builds?
- Are rigidbodies using interpolation?
- Are frame rates significantly different between test devices?
- Are objects desynchronized in multiplayer?
- Is physics auto-simulation enabled on one platform but not another?
Pro Tips
- Avoid real-time physics for critical gameplay logic; instead, use triggers or state machines where possible.
- Use Unity’s XR Interaction Toolkit or similar systems that abstract input and physics, helping maintain consistency.
- Regularly test builds on both high- and low-performance devices.